[ale] Mini pci card in laptops: compatibility?

[millia at panix.com (millia)]

So we have a need to put wireless on some older laptops. B/g, nothing  
too fancy, and it'd be preferable were it internal.
So they have a mini pci slot. About 40 pins, and not pci-e. Here's the  
question: is that a fairly defined standard, or are the cards fairly  
tied to a mobo/brand? I.e. Will any work, or do I need to spend double  
for the specific one for this unit?

Never done this before. Thanks!